1. Field of the Invention
Our present invention relates to a drawing unit for a drawing machine and to a method of drawing an elongated metallic workpiece and suitable for use in a drawing machine.
2. Background of the Invention
Metallic workpieces such as rods, bars and tubes can be drawn by applying a linear pull to the workpiece to draw through a die which reduces the cross section and thus provides a draft in the workpiece, and in part a shape or profile thereto.
Drawing machines of this type can include at least one drawing carriage with a drive for imparting a linear displacement to the carriage. The carriage can be displaced back and forth in a machine frame. The carriage may be provided with jaws capable of engaging and disengaging from the workpiece. The machine used for this purpose can operate continuously on the workpiece and thereby produce a workpiece at limited length or can be a discontinuously operating machine such as a drawing bench.
An example of a continuous linear drawing machine is the machine described in DE OS 28 52 071. This patent publication discloses a carriage-type drawing machine with a drawing unit having two drawing carriages displaceable in guides of a machine frame which run parallel to the drawing direction. The drawing carriages move counter to one another and, in their back and forth movement, draw the workpiece in an endless fashion by a hand-to-hand pulling of the workpiece through the die. The displacement of the carriages is effected by double-arm levers and a double-curved cam which displaces the same.
Another drive system which can be used is that of EP 0 371 165 A1 which has a rotating drum with a cam on its surface formed by a rib and producing opposite movements of two carriages in the back and forth sense.
The strokes of these carriages are relatively small and, as a consequence, the carriages are mostly either in an accelerating or braking mode. So that high mean drawing speeds can be achieved, therefore, they must operate at relatively high stroke frequencies. However, the frequencies at which they can operate is not unlimited since at the points at which reversal of travel direction occur, high inertial forces must be overcome.
It is, therefore, the principal object of the present invention to provide an improved drawing apparatus which can overcome the drawbacks of these earlier systems and permit optionally longer strokes of the carriages and higher mean carriage speeds than has been possible heretofore.
It is also an object of the invention to provide a drawing unit or apparatus which is free from drawbacks of prior art systems.
A further object is to provide a method of operating a drawing machine or an improved method of drawing elongated metallic workpieces whereby disadvantages of earlier techniques can be avoided.
These objects and others which will become apparent hereinafter are achieved, in accordance with the invention, in a drawing unit for a drawing machine capable of drawing an elongated metal workpiece and having at least one die through which the workpiece is drawn. According to the invention the drawing unit comprises:
a machine frame forming an elongated path along which the workpiece is drawn;
at least one drawing carriage provided with a workpiece gripper and displaceable back and forth linearly along the path to draw the workpiece through the die; and
a controlled linear motor complex assigned to the drawing carriage and acting upon the respective drawing carriage for independent displacement of the drawing carriage along the path.
The method of the invention can comprise displacing at least one drawing carriage of a drawing apparatus and preferably a plurality of drawing carriages by respective linear motor complexes so that the pattern of movement of the respective drawing carriage can be flexibly adjusted as required by control of the respective linear motor. In a preferred mode of operations, for each pull of the workpiece through the die, at least one pair of such carriages simultaneously engage and displace the workpiece under the control of the linear motor complexes driving the carriages of the pair.
Advantageously the drives for the respective linear motors are individually controlled and regulated and thus the individual linear motor complexes are individually controlled for the respective drawing carriages to provide movement patterns of the drawing carriages which are independent of one another.
By comparison with the drive system of the prior art, the movement pattern of the individual carriages are substantially more flexible. The linear motors allow the stroke lengths, the displacements and the locations of the strokes and displacements of the individual carriages along the path to be selected in a variable manner which is no longer limited by a fixedly located cam.
The linear motors can be controlled with precision and provide a rapid acceleration and rapid braking. They can provide especially long strokes, even where they are used to create back and forth movement of the carriage and thus can provide average drawing speeds which are significantly greater than those attainable heretofore since the acceleration and braking segments can be proportionately shorter. In addition, the respective carriages can have different and controllable speeds for the forward and rearward strokes. As a consequence, the displacement of a carriage backwardly to engage the workpiece can be greater than has been possible heretofore and thus the time consumed in the rearward travel can be reduced and the productivity of the apparatus enhanced. In addition, since a complex mechanical drive below the carriages is not required, the overall height of the drawing machine can be significantly reduced.
Preferably the linear motor complex for a pulling carriage or slide encompasses one or more linear motor units assembled in a modular fashion by appropriate selection of the number and/or drive force of the linear motor units. Starting from a basic carriage which either has no linear motor unit or a unit of low power, the power applied to a carriage can be built up by assembling additional motor units therewith.
In a first embodiment of the invention the primary part or primary parts of the linear motor unit or units constitute movable members on a carriage and are assembled in a modular construction whereas the secondary part or parts are stationary elements which are disposed along the machine frame as metal tracks or rails for the carriage. The guide of the carriage along the path may utilize separate guide rails.
In a preferred second embodiment there is kinematic reversal and the primary part or parts of the linear motor unit or units are formed as stationary elements along the machine frame whereas the secondary part or parts are movable elements connected to the respective carriage and displaceable along the primary parts. The secondary parts of the linear motor units are thus applied to the carriages while the primary parts are mounted on the machine frame.
The predrawing of the end of the workpiece which is required to enable the workpiece to be gripped by the jaws of the drawing carriages can be effected with a separate predrawing carriage which can have either a linear motor drive or a hydraulic drive, or as a combined predrawing and drawing carriage. The latter is preferably provided with a linear electric motor drive although it can have a mechanical, e.g. hydraulic, drive.
Such a combined predrawing and drawing carriage, referred to herein as a xe2x80x9ccombi-carriagexe2x80x9d, can have two clamping jaw pairs with the first pair engageable with the stem of the workpiece which is fitted through the die to commence the drawing process while the second pair engages the outer periphery of the workpiece, e.g. a tube behind this stem.
One drawing unit has only one such carriage, namely a combination predrawing and drawing carriage, forms the first unit for a basic unit for a drawing apparatus or line. A tube, for example, can be drawn with such a unit discontinuously utilizing the single carriage which can be displaced over a certain short stroke back and forth to allow tubes of relatively long length to be drawn. The base unit serves especially for an apparatus having a relatively low capacity and can be made at limited capital cost.
This basic unit can be expanded by providing two or more further driving carriages in conjunction with the combined predrawing and drawing carriage. In this case, the combi-carriage serves to predraw the workpiece in a first step and thereafter serves purely for drawing the workpiece in conjunction with the additional drawing carriages, for example in a hand-to-hand or continuous operation.
Another basic unit for a drawing line can have a separate predrawing carriage and two or more drawing carriages. This arrangement has the advantage that the predrawing carriage need have only one pair of clamping jaws which are designed to engage the leading end of the tube. A predrawing carriage of this type is of latter construction than a combi-carriage and permits a higher rate of acceleration. This system is also advantageous since all of the carriages, namely, the predrawing and the regular drawing carriages, need have only one pair of jaws each which limits the capital cost of the system.
In both cases, either the predrawing operation can utilize a basic carriage of modular construction with its own linear motor complex also of modular construction which can be assembled with either fewer parts or parts of smaller power than may be required for the regular drawing carriages, which must have higher drawing forces.
The drawing units of the invention can, of course, have more than two drawing carriages and can be so constructed and arranged that two or more drawing carriage simultaneously engage the workpiece during the drawing or pulling operation.
According to a feature of the invention, each drawing carriage is itself driven by a linear motor complex and the pattern of movement of the respective drawing carriage is flexibly controlled by the control, usually via a computer, of the respective linear motor complex. This permits a flexibility and variability of the drawing path and the functions of the individual carriages which has not been attainable heretofore.
From the method point of view, the movement pattern during drawing of an apparatus with more than two drawing carriages is so controlled that two or more drawing carriages simultaneously engage the workpiece and are displaced by their linear motor complexes in the drawing direction. Preferably at least one pair of drawing carriages is engaged with the workpiece for the drawing action. In this system, the pairs of carriages which are effected to draw the workpiece may be reformed by pairing off the different carriages while the unpaired carriage is displaced in a return movement. The pairing of carriages for the drawing operation allows multiplication of the drawing force. The drawing force thus is a function of the number of carriages which simultaneously engage the workpiece during the drawing operation and thus is doubled for a pair of carriages, tripled for three carriages, etc.
With the drawing apparatus having a total of there drawing carriages, therefore, the following drawing pattern may be used: drawing of the workpiece by two carriages while the third carriage is displaced in a return direction, recombining the carriages so that another pair then serves for drawing while a third carriage is returned and repeating the re-pairing in each case so that all of the carriages are paired with each other carriage for the drawing operation while another carriage is displaced in the return direction.
By contrast with the earlier drive systems which always required two carriages to move simultaneously in opposite directions, the linear motor complexes of the invention enable pairing of the carriages for joint drawing of the workpieces.
The invention can operate in a continuous drawing operation where each drawing stage utilizes a different pair of jaw. For a discontinuous drawing machine, here a drawing bench, the drawing unit can include a drawing chain into which the drawing carriage is connected. At the end of the pull, the carriage is released from the chain and returned to its starting position with a linear electric motor. For low drawing forces, the drawing chain can be eliminated entirely and both the forward and rear movements can be effected by the linear motor complex.